1,3-Propanediol is a monomer having potential utility in the production of polyester fibers and the manufacture of polyurethanes and cyclic compounds.
A variety of chemical routes to 1,3-propanediol are known. For example ethylene oxide may be converted to 1,3-propanediol over a catalyst in the presence of phosphine, water, carbon monoxide, hydrogen and an acid, by the catalytic solution phase hydration of acrolein followed by reduction, or from hydrocarbons such as glycerol, reacted in the presence of carbon monoxide and hydrogen over catalysts having atoms from group VIII of the periodic table. Although it is possible to generate 1,3-propanediol by these methods, they are expensive and generate waste streams containing environmental pollutants.
It has been known for over a century that 1,3-propanediol can be produced from the fermentation of glycerol. Bacterial strains able to produce 1,3-propanediol have been found, for example, in the groups Citrobacter, Clostridium, Enterobacter, Ilyobacter, Klebsiella, Lactobacillus, and Pelobacter. In each case studied, glycerol is converted to 1,3-propanediol in a two step, enzyme catalyzed reaction sequence. In the first step, a dehydratase catalyzes the conversion of glycerol to 3-hydroxypropionaldehyde (3-HP) and water (Equation 1). In the second step, 3-HP is reduced to 1,3-propanediol by a NAD.sup.+ -linked oxidoreductase (Equation 2). EQU Glycerol.fwdarw.3-HP+H.sub.2 O (Equation 1) EQU 3-HP+NADH+H.sup.+ .fwdarw.1,3-Propanediol+NAD.sup.+ (Equation 2)
The 1,3-propanediol is not metabolized further and, as a result,accumulates in high concentration in the media. The overall reaction consumes a reducing equivalent in the form of a cofactor, reduced .beta.-nicotinamide adenine dinucleotide (NADH), which is oxidized to nicotinamide adenine dinucleotide (NAD.sup.+).
The production of 1,3-propanediol from glycerol is generally performed under anaerobic conditions using glycerol as the sole carbon source and in the absence of other exogenous reducing equivalent acceptors. Under these conditions, in for example, strains of Citrobacter, Clostridium, and Klebsiella, a parallel pathway for glycerol operates which first involves oxidation of glycerol to dihydroxyacetone (DHA) by a NAD.sup.+ - (or NADP.sup.+ -) linked glycerol dehydrogenase (Equation 3). The DHA, following phosphorylation to dihydroxyacetone phosphate (DHAP) by a DHA kinase (Equation 4), becomes available for biosynthesis and for supporting ATP generation via, for example, glycolysis. EQU Glycerol+NAD.sup.+ .fwdarw.DHA+NADH+H.sup.+ (Equation 3) EQU DHA+ATP.fwdarw.DHAP+ADP (Equation 4)
In contrast to the 1,3-propanediol pathway, this pathway may provide carbon and energy to the cell and produces rather than consumes NADH.
In Klebsiella pneumoniae and Citrobacter freundii, the genes encoding the functionally linked activities of glycerol dehydratase (dhaB), 1,3-propanediol oxidoreductase (dhaT), glycerol dehydrogenase (dhaD), and dihydroxyacetone kinase (dhaK) are encompassed by the dha regulon. The dha regulons from Citrobacter and Klebsiella have been expressed in Escherichia coli and have been shown to convert glycerol to 1,3-propanediol.
Biological processes for the preparation of glycerol are known. The overwhelming majority of glycerol producers are yeasts, but some bacteria, other fungi and algae are also known to produce glycerol. Both bacteria and yeasts produce glycerol by converting glucose or other carbohydrates through the fructose- 1,6-bisphosphate pathway in glycolysis or by the Embden Meyerhof Parnas pathway, whereas, certain algae convert dissolved carbon dioxide or bicarbonate in the chloroplasts into the 3-carbon intermediates of the Calvin cycle. In a series of steps, the 3-carbon intermediate, phosphoglyceric acid, is converted to glyceraldehyde 3-phosphate which can be readily interconverted to its keto isomer dihydroxyacetone phosphate and ultimately to glycerol.
Specifically, the bacteria Bacillus licheniformis and Lactobacillus lycopersica synthesize glycerol, and glycerol production is found in the halotolerant algae Dunaliella sp. and Asteromonas gracilis for protection against high external salt concentrations (Ben-Amotz et al., Experientia 38, 49-52, (1982)). Similarly, various osmotolerant yeasts synthesize glycerol as a protective easure. Most strains of Saccharomyces produce some glycerol during alcoholic fermentation, and this can be increased physiologically by the application of osmotic stress (Albertyn et al., Mol. Cell. Biol. 14, 4135-4144, (1994)). Earlier this century commercial glycerol production was achieved by the use of Saccharomyces cultures to which "steering reagents" were added such as sulfites or alkalis. Through the formation of an inactive complex, the steering agents block or inhibit the conversion of acetaldehyde to ethanol; thus, excess reducing equivalents (NADH) are available to or "steered" towards DHAP for reduction to produce glycerol. This method is limited by the partial inhibition of yeast growth that is due to the sulfites. This limitation can be partially overcome by the use of alkalis which create excess NADH equivalents by a different mechanism. In this practice, the alkalis initiated a Cannizarro disproportionation to yield ethanol and acetic acid from two equivalents of acetaldehyde.
The gene encoding glycerol-3-phosphate dehydrogenase (DAR1, GPD1) has been cloned and sequenced from S. diastaticus (Wang et al., J. Bact. 176, 7091-7095, (1994)). The DAR1 gene was cloned into a shuttle vector and used to transform E. coli where expression produced active enzyme. Wang et al. (supra) recognize that DAR1 is regulated by the cellular osmotic environment but do not suggest how the gene might be used to enhance 1,3-propanediol production in a recombinant organism.
Other glycerol-3-phosphate dehydrogenase enzymes have been isolated: for example, sn-glycerol-3-phosphate dehydrogenase has been cloned and sequenced from S. cerevisiae (Larason et al., Mol. Microbiol. 10, 1101, (1993)) and Albertyn et al., (Mol. Cell. Biol. 14, 4135, (1994)) teach the cloning of GPD1 encoding a glycerol-3-phosphate dehydrogenase from S. cerevisiae. Like Wang et al. (supra), both Albertyn et al. and Larason et al. recognize the osmo-sensitivity of the regulation of this gene but do not suggest how the gene might be used in the production of 1,3-propanediol in a recombinant organism.
As with G3PDH, glycerol-3-phosphatase has been isolated from Saccharomyces cerevisiae and the protein identified as being encoded by the GPP1 and GPP2 genes (Norbeck et al., J. Biol. Chem. 271, 13875,(1996)). Like the genes encoding G3PDH, it appears that GPP2 is osmosensitive.
Although biological methods of both glycerol and 1,3-propanediol production are known, it has never been demonstrated that the entire process can be accomplished by a single recombinant organism.
Neither the chemical nor biological methods described above for the production of 1,3-propanediol are well suited for industrial scale production since the chemical processes are energy intensive and the biological processes require the expensive starting material, glycerol. A method requiring low energy input and an inexpensive starting material is needed. A more desirable process would incorporate a microorganism that would have the ability to convert basic carbon sources such as carbohydrates or sugars to the desired 1,3-propanediol end-product.
Although a single organism conversion of fermentable carbon source other than glycerol or dihydroxyacetone to 1,3-propanediol would be desirable, it has been documented that there are significant difficulties to overcome in such an endeavor. For example, Gottschalk et al. (EP 373 230) teach that the growth of most strains useful for the production of 1,3-propanediol, including Citrobacter freundii, Clostridium autobutylicum, Clostridium butylicum, and Klebsiella pneumoniae, is disturbed by the presence of a hydrogen donor such as fructose or glucose. Strains of Lactobacillus brevis and Lactobacillus buchner, which produce 1,3-propanediol in co-fermentations of glycerol and fructose or glucose, do not grow when glycerol is provided as the sole carbon source, and, although it has been shown that resting cells can metabolize glucose or fructose, they do not produce 1,3-propanediol. (Veiga D A Cunha et al., J. Bacteriol. 174, 1013 (1992)). Similarly, it has been shown that a strain of Ilyobacter polytropus, which produces 1,3-propanediol when glycerol and acetate are provided, will not produce 1,3-propanediol from carbon substrates other than glycerol, including fructose and glucose. (Steib et al., Arch. Microbiol. 140, 139 (1984)). Finally Tong et al. (Appl. Biochem. Biotech. 34, 149 (1992)) has taught that recombinant Escherichia coli transformed with the dha regulon encoding glycerol dehydratase does not produce 1,3-propanediol from either glucose or xylose in the absence of exogenous glycerol.
Attempts to improve the yield of 1,3-propanediol from glycerol have been reported where co-substrates capable of providing reducing equivalents, typically fermentable sugars, are included in the process. Improvements in yield have been claimed for resting cells of Citrobacter freundii and Klebsiella pneumoniae DSM 4270 cofermenting glycerol and glucose (Gottschalk et al., supra., and Tran-Dinh et al., DE 3734 764); but not for growing cells of Klebsiella pneumoniae ATCC 25955 cofermenting glycerol and glucose, which produced no 1,3-propanediol (I-T. Tong, Ph.D. Thesis, University of Wisconsin-Madison (1992)). Increased yields have been reported for the cofermentation of glycerol and glucose or fructose by a recombinant Escherichia coli; however, no 1,3-propanediol is produced in the absence of glycerol (Tong et al., supra.). In these systems, single organisms use the carbohydrate as a source of generating NADH while providing energy and carbon for cell maintenance or growth. These disclosures suggest that sugars do not enter the carbon stream that produces 1,3-propanediol. In no case is 1,3-propanediol produced in the absence of an exogenous source of glycerol. Thus the weight of literature clearly suggests that the production of 1,3-propanediol from a carbohydrate source by a single organism is not possible.
The problem to be solved by the present invention is the biological production of 1,3-propanediol by a single recombinant organism from an inexpensive carbon substrate such as glucose or other sugars. The biological production of 1,3-propanediol requires glycerol as a substrate for a two step sequential reaction in which a dehydratase enzyme (typically a coenzyme B.sub.12 -dependent dehydratase) converts glycerol to an intermediate, 3-hydroxy-propionaldehyde, which is then reduced to 1,3-propanediol by a NADH- (or NADPH) dependent oxidoreductase. The complexity of the cofactor requirements necessitates the use of a whole cell catalyst for an industrial process which utilizes this reaction sequence for the production of 1,3-propanediol. Furthermore, in order to make the process economically viable, a less expensive feedstock than glycerol or dihydroxyacetone is needed. Glucose and other carbohydrates are suitable substrates, but, as discussed above, are known to interfere with 1,3-propanediol production. As a result no single organism has been shown to convert glucose to 1,3-propanediol.
Applicants have solved the stated problem and the present invention provides for bioconverting a fermentable carbon source directly to 1,3-propanediol using a single recombinant organism. Glucose is used as a model substrate and the bioconversion is applicable to any existing microorganism. Microorganisms harboring the genes encoding glycerol-3-phosphate dehydrogenase (G3PDH), glycerol-3-phosphatase (G3P phosphatase), glycerol dehydratase (dhaB), and 1,3-propanediol oxidoreductase (dhaT), are able to convert glucose and other sugars through the glycerol degradation pathway to 1,3-propanediol with good yields and selectivities. Furthermore, the present invention may be generally applied to include any carbon substrate that is readily converted to 1) glycerol, 2) dihydroxyacetone, or 3) C.sub.3 compounds at the oxidation state of glycerol (e.g., glycerol 3-phosphate) or 4) C.sub.3 compounds at the oxidation state of dihydroxyacetone (e.g., dihydroxyacetone phosphate or glyceraldehyde 3-phosphate).